This invention relates to an improved layered aramid paper having a smooth-calendered surface and good tensile and tear strengths and low ink absorption so that the paper is suitable for printing over its entire surface. The smooth-calendered surface and low ink absorptivity provides better print clarity and makes such papers particularly useful for high temperature label applications even including those label printing applications that require high clarity and definition such as, for example bar coding. It is known in the art that aramid papers having 40-55% fibrids perform well in many uses. Prior art techniques that improve on surface smoothness often lead to a reduced level of mechanical strength and/or thermal stability: For example, surface smoothness can be improved by the increased concentration of fibrids in the paper, but while increasing the concentration of fibrids in the paper improves smoothness, the fibrids do not provide mechanical strength. When the fibrid content of the paper is high enough to provide a calendered surface suitable for high quality printing, the mechanical strength of the paper is diminished to such an extent the sheet is not usable in practice. Aramid papers, which do not have high fibrid content and mechanical strength and which have been pressed or calendered at high temperature and pressure will generally have fibers on the surface which cause roughness or snagging when the surface of the paper is worked during end use processing. This roughness can be measured as a surface abraded fiber count. Too great a surface roughness renders the surface of the paper unsuitable for high definition, high clarity printing.
In U.S. Pat. No. 4,888,091 to Nollen et al. an uncalendered sheet structure is disclosed that is useful for use in thermal and acoustical insulation. The sheet according to Nollen et al. is a commingled mixture of floc (short fibers) and fibrids. The sheet can be expanded to form air pockets after it is dried, but the sheet itself or some layered structure of these sheets is subject to the limitation on surface smoothness and mechanical strength as described above. That is as more fibrids are used to increase surface smoothness, mechanical properties of the paper decline rapidly to a point of practical limitation. Attempts to improve mechanical properties by expanding the wetted sheet structure render the surface unsuitable for printing by creating expanded portions which can be easily flexed.
The present invention is a single, consolidated paper formed of multiple layers each having a different compositions as to the weight percentage of fibrids and short fibers. The surface layer is constituted to provide a smooth-calendered nonpatterned surface having low ink absorption and being suitable for printing. The substrate layer, the layer supporting the surface layer or layers, is constituted to provide for mechanical properties. The surface and substrate layers are wet laid in such a way that they bonded at the interface of the layers such that the paper is a single, consolidated structure that can not be easily separated into the multiple layers from which it is formed. A sheet of paper according to the present invention can be pictured in its most simple form as having two layers which constitute a single sheet although in practice it is more common for a sheet according to present invention to have two surface layers and a substrate or even to be constituted of more than three layers. Considering then the most simple form of the invention having a surface and a substrate, the surface layer covering the substrate is fibrid rich when compared to the composition of the substrate layer. The substrate layer provides a fiber rich layer imparting mechanical strength. The process of wet laying one layer on the other forms the consolidated, unified structure. The surface layer composition is such that the paper can be calendered resulting in a paper surface which is not rough. The smooth-calendered surface of the paper according to the present invention provides a surface suitable for high quality printing. The thickness of each layer can be varied to produce a combination of surface smoothness and mechanical properties required for a particular printing purpose.